The successful working of this system of profitable extortion was dependent on two factors: the inability of the Muslim princelings to form lasting alliances amongst themselves and the consolidation of territorial gains by the Christian forces.
The existence of a Christian settler population, which moved into areas freed from the threat of Muslim attack, provided the necessary infrastructure to surviv~ any Muslim counter-attack. The tenth-century northern migration of Mozarabs (Arabized Christians) and the subsequent encouragement of colonists from north of the‘Pyrenees, provided a settler population which could consolidate the successes of the Christian raiding parties. Their presence in often dangerous regions was encouraged by the Spanish monarchs and this chiefly explains the prevalence of freeholdings in the countryside and the granting of town customs (fueros or cartas pueblas) which gave their populations a high degree of autonomy. In many areas, small or medium-sized estates predominated. Their owners, far from being restricted in their activities by feudal controls, often chose their own lords.
The demand for grazing land in the predom~nantlypastoral economy of the meseta gave a further impetus to territorial expansion and the inhabitants had a vested interest in keeping them safe from attack. The establishment of safe refuges in the towns was also vital. The newly captured cities, such as Salamanca, Guadalajara, and Avila-with its great granite wall~ and eighty-eight towerswere controlled by their own inhabitants and their fueros’reflected the importance of organization for both offensive and defensive warfare.